The present disclosure relates to burner assemblies, and particularly to oxygen-fuel burner assemblies configured to burn pulverized solid fuels. More particularly, the present disclosure relates to apparatus for mixing oxygen and fuel for use in a burner.
Many types of coal and other solid fuels can be burned successfully in pulverized form. Coal is pulverized and delivered to fuel-burning equipment and then combusted in a furnace to produce heat for various industrial purposes.
A burner is used to “fire” pulverized coal and other solid fuels. In a direct-firing system, the coal is delivered to the burner in suspension in a stream of primary air, and this mixture must be mixed with a stream of secondary air at the burner.
One challenge facing the burner industry is to design an improved burner that produces lower nitrogen oxide emissions during operation than conventional burners. Typically, an industrial burner discharges a mixture of fuel and either air or oxygen. A proper ratio of fuel and air is established to produce a combustible fuel and air mixture. Once ignited, this combustible mixture burns to produce a flame that can be used to heat various products in a wide variety of industrial applications. Combustion of fuels such as natural gas, oil, liquid propane gas, low BTU gases, and pulverized coals often produce several unwanted emissions such as nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHC).
According to the present disclosure, an apparatus is provided for combining oxygen and fuel to produce a mixture to be burned in a burner. The apparatus includes a fuel supply tube configured to communicate a stream of fluidized, pulverized, solid fuel to a “fuel-ignition zone” provided, for example, by a flame chamber formed in a refractory shape coupled to a downstream portion of the fuel supply tube. The apparatus further includes an oxygen supply housing coupled to an upstream portion of the fuel supply tube, an oxygen manifold coupled to the downstream portion of the fuel supply tube, and an oxygen distribution system for varying the amount of oxygen conducted to the oxygen supply housing and to the oxygen manifold.
In the illustrated embodiment, the oxygen supply housing cooperates with the upstream portion of the fuel supply tube to establish an oxygen-fuel mixer defining an upstream oxygen chamber adapted to receive oxygen provided by the oxygen distribution system. The upstream portion of the fuel supply tube is formed to include an upstream set of oxygen-injection holes opening into a fuel transport passageway located in the upstream portion of the fuel supply tube. Oxygen flows through those holes to mix with a fluidized, pulverized, solid fuel flowing through the passageway to produce an oxygen-enriched (yet not spontaneously combustible) oxygen-fuel transport mixture flowing toward the fuel-ignition zone in the flame chamber.
Also in the illustrated embodiment, the oxygen manifold is configured to communicate oxygen from the oxygen distribution system to the downstream portion of the fuel supply tube to produce a combustible oxygen-fuel mixture exiting the passageway to be ignited in the fuel-ignition zone to produce a flame. The oxygen manifold also is configured to communicate oxygen from the oxygen distribution system through one or more staged-oxygen bypass conduits to a portion of the flame outside the flame-ignition zone. Such “diversion” of combustion oxygen flow through the staged-oxygen bypass conduits to a region of the flame away from the root of the flame contributes to lowered nitrogen oxide emissions.
A control system associated with the oxygen distribution system is used to operate a first valve located to regulate oxygen flow to the upstream oxygen chamber and to operate a second valve located to regulate oxygen flow to the oxygen manifold. The control system provides means for operating the first and second valves to establish: (1) how much of the oxygen obtained from an oxygen supply is routed to the upstream oxygen chamber through the upstream set of oxygen-injection holes to mix with the fluidized, pulverized, solid fuel stream in the oxygen-fuel mixer and (2) how much of that oxygen is routed to the oxygen manifold for discharge through the downstream portion of the fuel supply tube and the flame chamber inlet to the “root” of the flame and for discharge through the staged-oxygen bypass conduit to the “tip” of the flame.
In one illustrated embodiment, an oxygen sensor is arranged to detect the amount of oxygen extant in the fluidizing gas to be mixed with the pulverized solid fuel. The control system is linked to the oxygen sensor provided and cooperates with the oxygen sensor to provide means for varying the amount of oxygen conducted through the oxygen distribution system to the oxygen-fuel mixer after determining an approximate concentration of oxygen in the stream of fluidized, pulverized, solid fuel. Such means can be used to maintain the concentration of oxygen in the oxygen-enriched (yet not spontaneously combustible) oxygen-fuel transport mixture produced by the oxygen-fuel mixer in the upstream portion of the fuel supply tube at a not spontaneously combustible level.
Additional features of the present disclosure will become apparent to those skilled in the art upon consideration of the following detailed description of illustrative embodiments exemplifying the best mode of carrying out the disclosure as presently perceived.